A system performance study on a portable life support system being developed for use in the Weightless Environment Training Facility (WETF) and the Neutral Buoyancy Laboratory (NBL) has been completed. The Neutral Buoyancy Portable Life Support System (NBPLSS) will provide life support to suited astronauts training for extravehicular activity (EVA) under water without the use of umbilicals. The basic configuration is characterized by the use of medium pressure (200 - 300 psi) cryogen (liquid nitrogen/oxygen mixture) which provides cooling within the Extravehicular Mobility Unit (EMU), the momentum which enables flow in the vent loop, and oxygen for breathing.NBPLSS performance was analyzed by using a modified Metabolic Man program to compare competing configurations. Maximum sustainable steady state metabolic rates and transient performance based on a typical WETF metabolic rate profile were determined and compared. The Systems Improved Numerical Differencing Analyzer-1985 (SINDA'85)/FLUid INTegrator (FLUINT) program was used to model the liquid cooling garment (LCG) vaporizer and the cryogen storage and supply tank. A Northern Research program was used to optimize pool heat exchanger designs.The system, as modelled, was able to sustain a maximum steady state metabolic rate of 1400 Btu/hr and maintain crew comfort. The success of the NBPLSS is dependent on the development of new designs to be used for the cryogen supply tank and the LCG vaporizer.